1. Field of the Invention
The invention generally relates to a field-effect transistor including an active layer having an oxide semiconductor, and a method for fabricating the field-effect transistor.
2. Description of the Related Art
The field-effect transistor (FET) is a transistor that controls current between a source electrode and a drain electrode based on the principle of applying a voltage to agate electrode to generate an electric field in a channel, whereby the gate is provided for the flows of electrons or holes. Based on the characteristics of FET, the FET is generally used as a switching device or amplifier device. The FET generally has a low gate current and a planar structure, which facilitate the fabrication or integration of the FET easier than a bipolar transistor. Accordingly, the FET is considered as one of the critical devices among the currently available integrated circuits in electronic devices. The FET may be applied as a thin film transistor (TFT) to an active matrix display.
Examples of a flat panel display (FPD) include liquid crystal displays, organic electroluminescent displays, and electronic paper. These types of FPD are generally driven by a drive circuit including a TFT having amorphous silicon or polysilicon as an active layer. Since the FPDs have increasingly been desired to have large sizes, high definition, and high driving rates, the TFTs need to have high carrier mobility, high ON/OFF ratios, and small device variability.
However, the TFTs having amorphous silicon or polysilicon as the active layer have their advantages and disadvantages, and hence it is difficult to simultaneously satisfy all the requirements. Moreover, the use of a flexible substrate such as a plastic film for the TFTs has been examined in view of fabricating displays having lightweight, high flexibility, and high impact resistance properties at a relatively low fabrication cost. In this case, however, since silicon needs to be treated at a relatively high temperature in a fabrication process, it is inappropriate to use silicon in the fabrication of TFTs in view of heat resistance of the substrate.
In order to satisfy the above-described requirements, numerous studies have been conducted and disclosed, for example, in U.S. Pat. No. 7,067,843 (hereinafter referred to as “Patent Document 1”), SCIENCE, VOL 300, 23, May, 2003, p. 1269-1272 (hereinafter referred to as “Non-Patent Document 1” disclosed by K. Nomura et al., “Thin-Film Transistor Fabricated in Single-Crystalline Transparent Oxide Semiconductor”, SCIENCE, VOL 300, 23, May, 2003, p. 1269-1272), and NATURE, VOL 432, 25, Nov. 2004, p. 488-492 (hereinafter referred to as “Non-Patent Document 2” disclosed by K. Nomura et al., “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”, NATURE, VOL 432, 25, Nov. 2004, p. 488-492) on the development of TFTs including oxide semiconductors having an expected carrier mobility higher than that of the amorphous silicon. Patent Document 1 discloses a TFT having ZnO for an active layer. Non-Patent Document 1 discloses a TFT having a single crystal InGaO3(ZnO)5 for a channel. Non-Patent Document 2 discloses a TFT having an amorphous In—Ga—Zn oxide for an active layer.
However, since the crystal structure of ZnO or In—Ga—Zn oxide is a hexagonal wurtzite structure or homologous series that exhibits a high anisotropy, it may be critical to control the orientation in the thin film. Accordingly, it is difficult to apply such a thin film to a large sized screen display.
Further, the amorphous In—Ga—Zn oxide has a property to easily undergo crystallization when Zn concentration is increased to achieve the high mobility.
Various studies suggest that a field-effect transistor having an oxide semiconductor mainly composed of magnesium (Mg) and indium (In) as an active layer eliminate the above drawbacks. Since the oxide semiconductor mainly composed of Mg and In has the crystal structure indicating its transportation characteristic being independent of the orientation of the thin film, the orientation of the thin film may not need to be controlled. Moreover, with such an oxide semiconductor mainly composed of Mg and In, high mobility and uniform characteristics may be achieved regardless of types (amorphous or crystal) of the oxide semiconductor.
However, evaluation of the field-effect transistor having the oxide semiconductor mainly composed of Mg and In as the active layer has shown that the active layer of the field-effect transistor is damaged while patterning the active layer by etching, which has eventually resulted in degrading field-effect transistor characteristics in an OFF-state.
Specifically, having compared the transistor characteristics before and after the patterning is carried out on the active layer by etching, the field-effect transistor after the patterning has exhibited degraded transistor characteristics, such as significant depletion characteristics and significant variability in the transistor characteristics between the samples.
An increase of current in on OFF-state is undesirable because it may induce the leak current or a decrease in the ON/OFF ratio. The significant depletion characteristics are also undesirable because it may require a gate voltage having a larger absolute value to switch the transistor to an OFF-state. Further, the variability in the transistor characteristics between the samples eventually results in the variability in device characteristics. That is, the above-described transistor degradation may result in the degradation of the display when the transistor is used as the drive circuit of the display.